We drain the head sink into the line that feeds flush water
to the head.
Our galley sink tees into our cockpit
The saltwater for our head comes from the engine
inlet cock ( but you need to be sure the engine
can't suck air back through this line).
For quite a while our cooling
water came from a hose dropped into a large cockpit
drain. Instead of a seacock you just pulled the hose out.
At the moment we have just one seacock for water
in, and another for toilet waste out.